A wide range of mechanical heart valves have been developed as substitutes for diseased natural heart valves. These mechanical heart valves are essentially check valves which operate hemodynamically in synchronization with the pumping action of the heart, allowing blood flow in a downstream direction and blocking flow in the opposite or upstream direction. Such mechanical valves may employ a single valve member or occluder, a pair of occluders or leaflets, or three or more leaflets, which are appropriately supported for pivoting or both pivoting and translational movement within a surrounding, supporting valve body or housing. An important feature of all of these prosthetic heart valves is the pivot or hinge mechanism that is employed which allows prompt and smooth opening and closing movement of the occluders to repeatedly occur.
One of the largest selling mechanical heart valves in the United States is that marketed by St. Jude Medical, Inc. which is described in U.S. Pat. No. 4,276,658 (Jul. 7, 1981). This bileaflet mechanical heart valve incorporates a pivot mechanism which utilizes pairs of recesses formed in diametrically opposed flat wall surfaces in a tubular valve body which has an otherwise circular cross section. Each of the leaflets is provided with a pair of laterally extending protrusions or ears that are received in these recesses. The recesses have surfaces in the form of sections of a sphere which serve as bearing surfaces, with flat transverse walls of the recesses serving as guides to control the angles of maximal opening and of closing of the leaflets.
Since the issuance of this patent in 1981, there have been a large number of bileaflet heart valve designs which incorporate recesses of some type in walls of a valve housing to contain and control the pivoting action of a pair of leaflets having edges that mate in the closed position. However, potential clotting is a constant concern in the operation of any mechanical heart valve, and one disadvantage of such a recessed pivot mechanism is that adequate washing of the entire recess becomes a major concern because the tendency of the mainstream flow of blood through the valve body is to bypass such offset recess regions.
To avoid the potential disadvantage of having to assure constant adequate washing of such recesses, a number of mechanical heart valves were created which employed pivots in the form of posts, bumps or other protrusions which extended outward from the interior wall of the valve body and thus would be exposed to the mainstream flow of blood through the valve body. Such arrangements take advantage of a positive washing effect at the surfaces of the pivot mechanism during both opening and closing of the leaflets. Examples of mechanical heart valves of this general type are found in U.S. Pat. Nos. 4,692,165, 4,822,353, 4,863,467, 5,080,669, 5,197,980, 5,171,263, 5,405,381, 5,354,330, 5,116,367, 5,080,669, 4,863,459 and 4,373,216. The last-mentioned patent to Klawitter employs a pair of leaflets with notches formed in their lateral edges which are received about projections that extend radially inward from flat wall surfaces of the valved body and guide the pivoting movement of the leaflets. Bumps or protuberances in the otherwise flat walls serve as stops which assist in positioning leaflets in the open and closed positions.
The advantages of procuring good washing of the pivot mechanisms as a result of the elimination of such recesses comes not without some disadvantage, namely the projection of the pivot or hinge structure into the mainstream flow of blood causes some obstruction to overall blood flow through the valve in the open position. Many such pivot mechanisms have surfaces that are essentially transverse to the flow of blood and have the tendency to cause separation of blood flow; such is undesirable from the standpoint of increasing the pressure drop across the valve and creating turbulent shear from such flow separation, the result of which can be damage to blood cells as well as thrombosis or emboli generation.
As a result, improvements in heart valve construction have continued to be sought with the objective of creating mechanical valves having flow characteristics such that there is minimum resistance to blood flow and/or damage to blood in the open position, and also having the capability to close promptly upon the termination of the pumping stroke with only little regurgitation.